1. Field of the Invention
This invention generally relates to a liquid crystal display apparatus and, more particularly, it relates to a liquid crystal display apparatus having a characteristic feature in the supply of video signals to the pixel electrodes of the circuit board.
2. Related Background Art
FIG. 1 of the accompanying drawings schematically shows a cross sectional view of a known liquid crystal panel. Since liquid crystal gives rise to a sticking phenomenon when a DC voltage is applied thereto, the polarity of the electric field being applied to the liquid crystal has to be inverted alternately on a field period by field period basis. For example, pixel M1 being subjected to an electric field that is directed upwardly from the pixel electrode toward the counter electrode in FIG. 1 in the current field period is then subjected to a downwardly directed electric field in the next field period to take over the current part of pixel M2 and then to an upwardly directed electric field in the following field period to resume the current part and so on. However, if the voltage applied to the liquid crystal is displaced from the proper polarity as shown in FIG. 2, a DC voltage is applied to the liquid crystal to make up the displacement to consequently give rise to a sticking phenomenon. Therefore, in order to avoid sticking, the counter potential has always to be located at the center of the amplitude of the pixel potential.
On the other hand, the brightness of a liquid crystal display apparatus depends on the absolute value of the electric field applied to the liquid crystal and does not relate to its polarity. Thus, if the counter potential is displaced from the center of the amplitude of the pixel potential, the electric field shows a difference in the absolute value of the electric field between when it is positive and when it is negative to become bright and dark alternately field period by field period. Referring to FIG. 4, if the counter potential is made to agree with the center of the incoming video signal typically by means of an oscilloscope arranged outside the liquid crystal panel, the field through caused by the gate capacitance generated, for example, when the gate of transistor T1 is turned OFF will change the potential of the pixel electrode M11 from that of the video signal written there and such a phenomenon can occur to all the pixels so that it is impossible to solve the above identified problem.
As a conventional technique of avoiding the problem of sticking and flicker, changes in the brightness of the liquid crystal of a liquid crystal panel that occur on a field period by field period basis is visually observed as flicker and the counter potential is regulated to eliminate such flicker. However, with such a technique, it is impossible to completely eliminate flicker from liquid crystal panels manufactured on a mass production basis because the extent to which the counter potential can be regulated is quite limited. With another known technique, a short field period or frame period is used to dissolve the problem of flicker. A very high frequency flicker appears to be smoothed to the eye and would no longer recognized as flicker. Generally, no flicker is noticed by the human eye when the frame rate is 70 Hz or above. Therefore, with this technique, flicker will be visually eliminated if the counter potential is displaced slightly from the center of the pixel potential.
However, there are cases where the problem of sticking cannot be solved by the technique of reducing the field period or the frame period. On the other hand, it is often difficult to correctly regulate the counter potential because the human eye cannot recognize flicker.
In view of the above identified problems, it is therefore the object of the present invention to provide a liquid crystal display apparatus having a simple configuration and adapted to dissolve the above problems.
According to an aspect of the invention, the above object is achieved by providing a liquid crystal display apparatus having a circuit substrate carrying thereon a plurality of pixel electrodes, a counter substrate carrying thereon a corresponding counter electrode disposed vis-a-vis the pixel electrodes and a liquid crystal layer held between the circuit board and the counter substrate and adapted to be controlled by the voltage applied between the pixel electrodes and the counter electrode, characterized in that it comprises a field frequency conversion means for multiplying the field frequency of the video signals input to the respective pixel electrode by a (a greater than 1) and a switching means for changing over from the inversion of the polarity of the video signal output from said field frequency conversion means by every field to the inversion of the polarity of the video signal output from said field frequency conversion means by every n fields (n being a positive integer not equal to 1) or vice versa.
Preferably, a liquid crystal display apparatus according to the invention further comprises a polarity inverting circuit for inverting the polarity of the video signal input to each of the pixel electrodes, an inversion period control means for controlling the timing of inversion of said polarity inverting circuit and a switching means for changing over from the inversion of the polarity of the video signal output from said field frequency conversion means by every field to the inversion of the polarity of the video signal by every n fields or vice versa.
According to another aspect of the invention, there is provided a liquid crystal display apparatus having a circuit substrate carrying thereon a plurality of pixel electrodes, a counter substrate carrying thereon a corresponding counter electrode disposed vis-a-vis the pixel electrodes and a liquid crystal layer held between the circuit board and the counter substrate and adapted to be controlled by the voltage applied between the pixel electrodes and the counter electrode, characterized in that it comprises a frame frequency conversion means for multiplying the frame frequency of the video signals input to the respective pixel electrode by a (a greater than 1) and a switching means for changing over from the inversion of the polarity of the video signal output from said frame frequency conversion means by every frame to the inversion of the polarity of the video signal output from said frame frequency conversion means by every n frames (n being a positive integer not equal to 1) or vice versa.
According to the invention, the period of the flicker caused by a displacement of the counter potential can be doubled or more than doubled by transforming the input video signal. Additionally, sticking and other problems of the liquid crystal of a liquid crystal display apparatus can be prevented from occurring by applying the driving circuit of a projection type liquid crystal display apparatus to it to prolong the service life of the liquid crystal display apparatus.